More often that not password issues with mail indicate problems with the anti virus program, not an issue with either the mail server or the mail client. This is exacerbated by using secure connection methods to access the server. Some Anti virus product simply give up. Other try and hack the connection, with varying results. What product do you use?

I use Avast.

If you believe it's the cause of the problem, why does it work for months then like clockwork stop working. I read that yahoo.com will be discontinued as a domain soon and that is the cause of my problem? I really don't want to lose the email as it covers my important online connections.

Bikenutter said

I use Avast. If you believe it's the cause of the problem, why does it work for months then like clockwork stop working.

Why do folks hearts work like clockwork for decades and then one day just stop? I know that sounds trite, but it is really about the best analogy I can come up with. It is accurate.

I read that yahoo.com will be discontinued as a domain soon and that is the cause of my problem?

Unlikely, a domain is either there or it is not. just like a fence or a post. Just as you can not be a little bit pregnant, a mail server can not be a little bit there.

I really don't want to lose the email as it covers my important online connections.

I don't follow that logic at all.

Yahoo have stumbled from one disaster to another. They have no apparent direction, not much in the way of skill in anything but web mail. So those that choose to use them get a rough ride.

As for your mail in the account, back it up.

https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Install that add-on and then when you right click on a folder there is an export option. Export your mail and move to a provider that consistently provides good email... GMX in Germany have an excellent reputation in that regard.

Install an add-on instructions here https://support.mozilla.org/en-US/kb/forum-response-installing-extension

After all that I forgot to address the original question.

To diagnose problems with Thunderbird, try the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.